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mHż kJ '94 13:23 BBNCC MET AMAL/.?S-ENG/PGM_OPS 
Extensions of packet communication technology 
to a hand'held personal terminal 
INTRODUCTION 
Electronic communications technology has developed 
historically almost completely within what might be 
ea!bd the circuit switching domain. Not until the last 
decade has the other basic mode of communication, 
packet switching, become competitive. Thus, as a tech- 
nology, packet communication has only begun to be 
explored. Circuit switching can be defined in the broad 
sense as the technique of establishing a complete path 
between two parties for as long as they wish to com- 
municate, whereas packet switching is where the com- 
munication is broken up into small messages or packets, 
attacking to each packet of information its source and 
destination and sending each of these packets off inde- 
pendently and asynchronously to find its way to the 
destination. In circuit switching all conflicts and alloca- 
tions of resources must be made before the circuit can 
be established thereby permitting the traffic to flow 
with no conflicts. In packet switching there is no dedi- 
cation of resources and conflict resolution occurs during 
the actual flow perhaps resulting in somewhat uneven 
delays being encountered by the traffic. Glearly, without 
the speed and capability of modern computers, circuit 
switching represented a cheaper and more effective 
way to handle communications. For radio frequency 
assignment and telephone exchanges the resource allo- 
cation decisions could be made infrequently enough 
that manual techniques were originally sufficient. Abe, 
since voice was the main information being communi- 
cated, the traffic statistics were sufficiently compatible 
with this approach to raake it quite economic for the 
period. Packet switehi-ri of a kind, the telegram, per- 
sisted throughout this period but due to the high cost 
of switching and the limited demand for fast message 
traffic never attracted much attention. 
For almost a century circuit switching dominated 
the communications field and thus dominated the de- 
velopment of communications theory and tcchnology. 
295 
Now, within the last decade or less, the advances in 
digital computers and electronics have, in many cases, 
reversed the economic balance between circuit and 
packet communication technologY' Perhaps the best 
proof of this is the economy of the ARPA Network :-8 
for country-wide computer to computer communica- 
tion, but many other examples are beginning to appear 
such as the University of Hawaii's ALOHA. System 7 
utilizing packet radio transmission for console com- 
munications and the experiments with digital loops for 
local distribution. However, most of the experiments 
with packet communications have been undertaken by 
computer scientists, and it is not even generally recog- 
nized yet in the communications field that a revolution 
is taking place. Even where the knowledge of one of 
these experiments has penetrated the communications 
field, it is generally written off as a possibly useful new 
twist in communications utilization, and not recognized 
as a very different technology requiring a whole new 
body of theory. Throughout the development of the 
ARPA Network, communication engineers compared it 
with conventional circuit switched systems but, per- 
haps unconsciously, used rules of thumb, statistics and 
experience applicable only to circuit switched systems 
as a basis for comparison. A century of experience and 
tradition is not easy to ignore and in fact should not be 
ignored, only it should be properly classified and megre- 
galed as resulting from a different technology. 
Packet communication technology is only beginning 
to be explored but already it is clear that the design of 
'all forms of communications channels and systems 
should be rethought. As an example of the kind of 
difference packet communications can make in a per- 
haps unexpected area, the design of a personal trminal 
will be explored in some detail. Although such a ter- 
minal has never been built, it is most likely completely 
feasible to build and would provide many unique 
advantages. 
:."-"'':'"Y 20 '94 1:3:24 BBNCC MET ANA 
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296 
Spring Joint Computer Conference, 1972 
HAND HELD PERSONAL TERMINAL 
Let us start Sth the goal of providing each individual 
with a pocket-sized, highly reliable and secure com- 
munications device wkioh would permit him to send 
and receive messages to other individuals or to eo- 
putera. Leaving the consideration of design alternatives 
muntil the end, a device fulfilling these objectives is as 
follows: 
Text or graphics displayed on a 2.8"X1" plasma 
panel with 80 dots per inch resolution. The screen, 
divided into 7 X 10 dot rectangles, using 5 X 7 characters 
would hold 8 lines of 32 characters each for a total of 
256 characters. Text this size is almost the same size as 
typewriter print, except that the lines are closer to- 
gethpr. The plasma panel has internal storage and is 
digitally addressed to write or erase a point. 
Inpug 
Five capacity or stress sensitive buttons used by the 
five fingers of one hand simultaneously to indicate one 
of 31 character§. This five finger keyboard technique 
was developed by Doug Eaglebart at SRP to permit 
users to type with only one hand while working on a 
display console. Recently the keyboard has become 
fairly widely used at SRI due to its great convenience. 
Training time for a new user is evidently less than a 
day and speeds of 30 words per minute can be achleved2 
Although somewhat slower than a good typist (/ 
speed) 'the speed is clearly sufiqcient for a terminal 
device even at 10 words/minute. 
Trarmision 
Each input character will be transmitted to a central 
controller station using the random access radio trans- 
mission techniques develop_eL at the University of 
▀ Hawaii. 7 The 5 bit char'te:is.embodied in a 64 bit 
- packet containing: : 
':30 bits--Terminal Identification Number 
"8 bits-Character plus alternation bit, or Count 
2 bitsType of packet (CHAR, ACK, CNT, 
ERR, ST ERR) 
24 bits--Cyclic Sum Check 
.'" 64 bits 
All terminals transmit their packets independently and 
asynchronously on a single frequency and the receiver 
at the central controller merely listens for a complete 
packet which has a correct sum check. If two terminals' 
transmissions overlap the sum check will be wrong, and 
the terminals will rctransmlt when they find they don't 
receive an acknowledgment. Retransmission time-out 
intervals are randomized between the terminals to 
avoid recurrence of the problem. Upon receipt of a good 
packet, the central station transmits a display-ac- 
knowledgment packet back to the terminal on a second 
frequency. This 144 bit packet contains a 70 bit display 
raster field and an 8 bit position on the screen. The dis- 
play raster is a 7X10 dot array for the character sent 
in and the position includes 3 bits for vertical by 5 bi 
for horizontal. Current position information for each 
active user is kept by the central station by user ID in 
a hash table. Thus, the individual terminal needs no 
character generation logic, position advancement logic, 
or any other local verification of the input since the 
response from the centrat station both acknowledges the 
character and displays it in an input text line at the top 
of the display. If a character display-acknowledgment 
is somehow lost in transmission the terminal will con- 
tinue to time-out and rctransmit the character. The 
central station must somehow differentiate this from 
a new character. This is achieved by an alternation 
bit °'a in the terminal's packet which is complemented 
for each new character. On a repeat the bit is the same 
as previously and the central station just retransmits 
the same character and position again. When a pre- 
arranged terminating character is sent the central 
station examines the masage and takes an appropriate 
action. Considerable flexibility exists here, and opera- 
tional modes could be established. However, the first 
message of a sequence should contain a dcstinatlon as 
the first item. This might be the ID of another terminal 
in the same area, it might be the address of a service 
computer or it might be the ID of another terminal 
halfway around the world. In the latter two cases, a 
more global network such as the. ARPA Network comes 
into play. It would be pe'rfcctly feasible for a message 
to another terminal to be sent to a central or area-coded 
directory computer to locate the particular control 
station the other terminal was near. Note that the loca- 
tion of neither man wsa given to the other, only the 
message and the ID of the sender. (Based on ARPA 
Network cost estimates and international satellite tariff 
trends, such a message exchange should cost less than 
0.1 cents, independent of distance.) 
Recepti<m 
At any time when a message destined for a terminal 
arrivc at the central control station, a transmission to 
....  ': ? BBNCC MET APIAL/SYS_Ei"IG/PGM_OPS r 
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Extensions of Packet Communication Technology 297 
the. terminal may begin, character by character, each 
in its own 144 bit packet as follows: 
30 blts--Tcrminal Identification Number 
70 bits--7X 10 dot pattern, character display 
8 bits--position of character ' 
1 bit maltcrnation bit 
▀ 1 bit --broadcast mode 
3 bite--Message Type (Response,' initial, 
normal) 
8 bits--Characters Left in message 
24 bits--Cyclic Sum Check 
144 bits 
-The terminal must always be checking all transmission 
to detect those with its ID and a correct sum check. 
When one occurs which is not a "response" to an input 
character, a message is being sent. The first character 
of a message is marked type "initial," and has the 
count of the remaining characters. Each character is 
displayed where the central station placed it. Following 
the "initial" character "normal" characters are checked 
to make sure the count only decreases by one each time. 
After the character with count zero', an acknowledg- 
ment type packet is sent by the terminal. If this is lost 
(s it may be due to conflicts) the central control will 
rctransmit the final character over again without com- 
plementleg the alternation bit until it is acknowledged 
(or it determines the station is dead). If a count is 
skipped the terminal sends a CNT ERR message with 
the ount of the character expected. The transmitter 
then star over at that count. If a "normal" type char- 
acter is received before an "initial" type a ST ERR 
message is sent and the message is restarted. A broad- 
cse bit is included which overrides the ID check for 
general messages. 
$ecurdy 
Since all transmissions arc digital, encryption is pos- 
sible and would bei_m_--_potant no matter what the appli- 
cation, military or civilian. Most private uses such s 
personal name file., "income-expense records, family 
conversations, etc., would be far more sclmit[vc than 
current computer console use. 
Bandwidth 
Personal terminals for occasional use for message ex- 
change, maintaining personal files, querying computer 
data bases for reference data, etc., would not lead to 
very heavy use, probably no more than two query- 
responses per hour. The query wc might mtimate at 64 
characters in length and the response at 256. (Clearly 
256 character response could also consis of an 80X 
224 point graphic display since each character is sent as 
a full 7X 10 raster.) The average bandwidth consumed 
by each terminal is therefore 2.3 bits/second trans- 
mitted and 25.6 bits/second received. The random ac- 
cess technique used for transmission requires the chan- 
nel bandwidth to be six times the average bandwidth 
actually utilized in order to resolve all conflicts prop- 
erly. Thus, the terminal transmission bandwidth con- 
sumption is 14 bits/second, still less than the receiver 
bandwidth needed. Thus, the central control station's 
transmitter bandwidth is the limiting factor assuming 
equal bandwidths on both transmitter and receiver. If. 
a 50KHz bandwidth is used for each and modulated 
at 50K bits/see, then a total of 2000 terrainsis can be 
accommodated. Of course this number depends on the 
activity factor. At one interaction every two minutes a 
data rate equal to average time shared console use is 
obtained and even at this activity 130 terminals can be 
supported, more than most time-sharing systems can 
support. With 50 KB channels, the time required to 
write 256 characters is about one second. Lower band- 
widths require increased time, thus, IOKB (5 sec write 
time) would be the lowest bandwidth reasonable. Even 
at this bandwidth, with the estimated 2 interactions 
per hour, 400 terminals could be supported. ' 
COMPARISON 
Comparing the effect of the packet technology with 
the same tcrminal operating with presssigned Fre 
quency or Time Division Multiplexed channels (ignor- 
ing the losses due to TDM sync bits or FDM guard 
bands) the circuit oriented terminal would require a 
40 bit/see transmit channel and a 4KB receive channel 
if a 5 sec write time is to be achieved. For 400 terminals 
with a 5 sec write time, the circuit method would re- 
quire a total of ].6 egabits/sec bandwidth whereas the 
packet method only requires 20 Kilobits/see band- 
width. Thus, the circuit technology requires a factor of 
80 more bandwidth than the packet technique. Of 
course, the circuit mode terminals could interact more 
often within the same bandwidth right up to continual 
rewrite of the display every live see, but you would 
have to massively reshape the user statistics to suit the 
technology. 
Another possibility, to design the terminal so that it 
performed moro effectively in a circuit oriented mode, 
would be to put character generation logic and position 
logic in the terminal. This would considerably increase 
the cost of the terminal, which originally had very little 
logic except shift registers. The result of adding this 
logic, however, is to reduce the bandwidth by a factor 
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298 
Spring Joint Computer Conference, 1972 
of i0 to .16MB or still 8 times the packet technique. 
The same logic would help reduce the packet size but, 
in order t) maintain the graphic output capability and 
gross simplicity, it does not seem to pay. 
CONCLUSION 
As can be see. n from the example, packet technology is 
far superior to circuit technology, even on ttxc simplest 
radio transmi.sion level, so long as the ratio of peak 
bandwidth to average bandwidth is large. Most likely, 
the only feasible way to decsign a useful and economi- 
cally attractive personal terminal is through some type 
of packet communication technology. Otherwise one is 
restricted to usele.sly small numbers of terminals on 
one channel. This result may also apply'to many other 
important developments, only to be discovered as the 
technology of pack(;t communication is further 
developed. 
REFERENCES 
L G ROBERTS B D WESSLEIt. 
Computer network development to achieve reSource sharing 
SJGC 1970 
F E I[EAI{.T It. E KAIIN S M OF(NSTEtN 
W R CROWTHER D C WALDEN 
The irderface messv, qe processor for the ARPA network 
SJCC 1970 
3 L KLEINROCK 
Analytic and simulation methods in compu& n:fwork design 
SJCC 1!170 
4 H FRANK I T FIt, ISCI W CIIOU 
Topolokal conzid'atioa, in lhc design of lha ARPA 
compulcr nclwork 
SJCC 1970 
5 S CARI{ S CROCKFA{ V CERF 
HOST-HOST commtmicalion prolocol in lhe ARPA nelwork 
SJCC 1970 
6 L O 11.OBF, RTS 
A .forward look 
Signal Vol XXV No 12 pp 77-81 August 1971 
7 N ABI.AMSON 
THE ALOHA System--Another' alto'native for comput' 
cornmunicalio 
AFIPS Conference Proceedings Vol 37 pp 281-285 
November 1970 
8 1) C ENGELBART W' K ENGLISII 
A research center for aumcnlinq human inlcllext 
AFIPS Conference Proceedings Vol 33 p 397 1968 
9 D C ENGELBAR.T 
Stanford Research Institute Menlo Park Calif (Personal 
communication) 
10 W C LYNCH 
Reliable full-dupl file transmission over half, duplex 
telephone lines 
mmunieation$ of the AC Vol 11 No 6 pp 407-419 
June 1968 
11 K A BARTLETT It A SCANTLEBURY 
P T WILKINSON 
A aa on edible full-duplx transmission ot, er half-duplex 
links 
Communications of th ACM Vol 12 No 5 pp 260-261 
May 1969 
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. /'J '94 13:8 BBMCC HET HLXSYS_ENgXPGM-OS 
COMMUNICatioNs 
ChaNNELS 
tEChNOLOGY 
1 
PACKETS 
▀ END-TO'END CIRCUIT 
▀ INFREQUENt DECISIONS 
▀ ADDRESSED MESSAGE 
eDYNaMIC rESOUrCE 
aLLOCatiON 
▀ TELEPHONE ('876) 
▀ .AD,O (,go,) 
▀ FACSIMILE (1924) 
AND DESCENDANT5 (lg60'S) 
▀ ARPANI:'I' (lg6!)) 
I I I I 
LGR/WHAT WILL THE DISPLAY HOLD?/ 
- THE PLASMA PANEL WILL DISPLAY 
8 LINES OF 32 CHARACTERS FOR A 
TOTAL OF 250. EACH CHARACTER l$ 
7x10 RASTER UNITS' ALTERNATIVELY 
THE 80x224 RASTER COULD BE USED 
FOR GRAPHICS OUTPUT, <LGR>' 
HAND HELD PERSONAL TERMINAL 
I I I I II I I III I I 
------------------------------<page break>-----------------------------
TERMINAL PACKET FORMAT 
[,D i-I C'"j c 'J 
BITS 
30 -' TERMINAL IDENTIFICATION 
2 -- PACKET TYPE 
8 -- CHARACTER '+ALT. BIT 
OR 
LAST GOOD COUNT 
24" ' CYCLIC SUM CHECK 
64. BITS TOTAL 
CENTRAL CONTROL PACKET FORMAT 
I "::' I 1_[ x,,, 17xo ,::,,,:,L_^',, o J 
BITS 
30 "' TERMINAL IDENTIFICATION 
5 . PACKET TYPE 
8 ----' NO, CHARACTERS LEFT 
8 - XY POSITION 
70 -- CHAR. DISPLAY DOT MATRIX 
24 CYCLIC SUM CHECK 
144 bits tOtaL 
y 
Mż 2 '94 13:30 BBHCC MET AHAL/ vc  ........ 
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PERFORMANCE WITH 80 KB BANDWIDTH 
looo 
too 
IO 
NUMBER OF 
TERMINALS 
PACKET 
SYSTEM/ -'; o SEC - 
-... /CHANNEL 
1 SEC 
USEFUL RANG[.' MESSAGES/HOUR 
lO I00 1000 
------------------------------<page break>-----------------------------
BANDWIDTH REQUIREMENTS FOR IOOO TERMINALS 
lOMB 
IMB 
IOOKB 
TOTAL 
BANDWIDTH 
PACKE-F 
SYSTEM 
Ol SEC 
SEC 
lo SEC 
I SEC 
CHANNEL 
SYSTEM 
SEC REWRITE 
USEFUL RANGE i MESSAGES/HOUR 
IOKB I I I 
I 10 loo lOOO 
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MAY 'P '94 l-'Ii lf'lCC: blET AbtALxSVS_EHGxPGM_CPS 
PERFORMANCE With -< 5 SEC DISPLAY REWRITE 
FOR A RATE OF 2 MESSAGES/HOUR 
IOOMB 
lOMB 
1MB 
100KB 
10KB 
TOIAL 
BANDWIDTH 
PaCkET 
SYSTEM 
NUMBER OF 
TERMINALS 
! 10 I00 1K 10K lOOk 1M 
CHANNEL 
SYSTEM 
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